1. Technical Field
The present invention relates to a forward converter to synchronously rectify an output current.
2. Background Art
A conventional synchronous rectifying forward converter is disclosed in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2000-262051). FIG. 1 shows the circuit of the converter according to Patent Document 1.
In the circuit shown in FIG. 1, after a main switch element 2 on the primary side of a transformer 4 is turned ON, a voltage generated at a secondary winding 4b of the transformer 4 causes a rectifying-side synchronous rectifier element 5 on the secondary side to be turned ON and a commutating-side synchronous rectifier element 6 to be turned OFF. If the turn-OFF of the commutating-side synchronous rectifier element 6 delays, a short circuit path including the two synchronous rectifier elements 5 and 6 is formed. Thus, a driving switch element 7 is provided in series with a tertiary winding 4c of the transformer 4 and the driving switch element 7 is tuned ON at the timing of turning ON the main switch element 2 on the primary side. With this configuration, a charge due to the gate capacitance of the commutating-side synchronous rectifier element 6 is discharged via the driving switch element 7 just before turn-ON of the main switch element 2 on the primary side, and the commutating-side synchronous rectifier element 6 is quickly turned OFF, so that a short circuit is prevented.
However, in the synchronous rectifying forward converter of so-called indirect control type shown in FIG. 1, which controls the synchronous rectifier elements 5 and 6 by using a winding voltage of the transformer, the following problem occurs. That is, when a switching operation of the main switch stops or when a relatively high voltage is applied to an output terminal, self-excited oscillation occurs on the secondary side and electric power flows back (regenerates) from the secondary side to the primary side.
Also, since the commutating-side synchronous rectifier element 6 is controlled by ON timing of the main switch element 2, the following problem occurs. That is, when a switch element driving circuit 12 stops, the commutating-side synchronous rectifier element 6 is not turned OFF and is kept in an ON-state until it is turned OFF due to current leakage between the gate and source of the commutating-side synchronous rectifier element 6. Therefore, if an overvoltage is applied (flows back) from the output side, a choke coil 8 on the secondary side is excited to cause self-excited oscillation.
The rectifying-side synchronous rectifier element 5 is turned ON by being supplied with the gate voltage thereof from the transformer 4. In a case where excitation of the choke coil 8 on the secondary side is performed from the primary side, when the excitation of the choke coil 8 ends at turn-OFF of the main switch element 2, the voltage thereof is inverted and thus the rectifying-side synchronous rectifier element 5 is turned OFF. In a case where excitation of the choke coil 8 is performed from the output side, the synchronous rectifier element 5 is turned OFF by reset of the excitation state of the choke coil 8 and inversion of the voltage. Accordingly, there exists a mode where the ON-period of the transformer 4 from the secondary side is controlled in accordance with the excitation state of the choke coil 8. The switch element driving circuit 12 tries to stabilize an output voltage by performing PWM (pulse width modulation) control on the main switch element 2 in accordance with the voltage obtained by rectifying and smoothing the winding voltage of the transformer 4. However, occurrence of the above-described backflow causes an increase in the winding voltage of the transformer 4, so that the control by the switch element driving circuit 12 stops. Even if the control by the switch element driving circuit 12 does not stop, control using the above-described winding voltage of the transformer and control using the choke coil 8 are mixed, which causes abnormal oscillation.
Furthermore, the circuit operation becomes unstable due to excitation of the choke coil 8 and the ON-period of the synchronous rectifier elements 5 and 6 is not controlled, so that disadvantageously a reverse current from the output side cannot be absorbed.